1. Field of the Invention
This invention relates to a dry etching employed in the preparation of semiconductor devices. More particularly, it relates to etching of a layer of an aluminum (Al) based material in which resist selectivity and underlying layer selectivity may be improved and maintenance of an etching apparatus may be facilitated.
2. Description of the Related Art
As metallization materials for semiconductor devices, aluminum (Al), an Al-Si alloy containing 1 to 2 wt % of silicon (Si) and an Al-Si-Cu alloy further containing 0.5 to 1 wt % of Cu for combatting stress migration, are extensively employed.
Dry etching of the layer of the Al-based material is usually performed using chlorine based gases. Typical of these gases is a BCl.sub.3 /Cl.sub.2 mixed gas as disclosed for example in JP Patent KOKOKU Publication 59-22374 (1984). Since chemical species contributing to the etching of the layer of the Al-based material as a main etchant are Cl* (Chlorine radicals), it inherently suffices to use only Cl.sub.2. However, native oxides are present on the layer of the Al-based material and, unless these oxides are reduced, etching does not proceed promptly. Besides, etching proceeds isotropically with the use of only Cl*, contribution of ions in some form or other is required in order to effect anisotropic etching. BCl.sub.3 is added in this consideration. That is, BCl.sub.3 generate BCl.sub.x, a chemical species exhibiting a strong reducing action, in a plasma, so that the native oxides on the surface of the Al-based material layer may thereby be reduced to cause the etching reaction to proceed smoothly. On the other hand, since ions derived from BCl.sub.3, such as BCl.sub.x.sup.+, promote forward sputtering of the resist mask to deposit carbonaceous decomposition products on the pattern sidewall, anisotropic etching may also be achieved by the sidewall protection effect offered by these deposits.
However, the BCl.sub.3 /Cl.sub.2 mixed gas suffers from the following problems.
The first problem is worsened maintenance. Since BCl.sub.3 is reacted with oxygen during evacuation to produce boric acid (B.sub.2 O.sub.3) which as white powders may possibly clog a piping of an evacuation system of the etching apparatus to prolong maintenance and to lower the throughput.
The second problem is that, since BCl.sub.3 exhibits strong reducing properties, an underlying insulating film composed of e.g. silicon oxide, if exposed, is reduced, and hence is susceptible to etching by Cl*. This indicates significant deterioration in the underlying layer selectivity during overetching.
The third problem is that, while an ion incident energy needs to be increased to some extent in expectation of forward sputtering of the resist mask, resist selectivity is then naturally lowered, and that, if the underlying layer is exposed and sputtered by ions having such a large energy, the sputtered particles of insulating film are deposited on the pattern sidewall to form a re-deposited layer. This re-deposited layer is porous and hence ready to occlude residual chlorine to promote after-corrosion. Since it is the recent practice to provide a barrier metal layer below or to add Cu to the Al-based material layer, thus in a manner unfavorable from the standpoint of preventing the after-corrosion, formation of the re-deposited layer which can possibly provide a site for residual chlorine needs to be avoided to the utmost extent.